Various techniques have hitherto been proposed in relation to a droplet discharge device for discharging a small amount of liquid material from a discharge opening by using a plunger that is moved reciprocally.
As an example of a droplet discharge device of the type hitting a tip of a plunger against a valve seat to discharge a liquid material in a droplet state, Patent Document 1 discloses a droplet discharge device for causing the liquid material to land on a workpiece after a droplet has departed from a nozzle, wherein a plunger is disposed in a flow path including a valve seat near an outlet of the flow path, the outlet communicating with the nozzle, such that a lateral surface of the plunger is not contacted with an inner wall of the flow path, and the tip of the plunger is moved toward the valve seat and is hit against the valve seat, thereby discharging the liquid material in the droplet state from the nozzle.
However, hitting the plunger against the valve seat raises the problems that the shape of the plunger is changed due to wear, and that wear dust or wear debris generates and contaminates the liquid material, or comes into a gap between the plunger and the valve seat, whereby satisfactory discharge is impeded.
In view of those problems, the applicant has proposed, as a droplet discharge device of the type discharging the liquid material without hitting the tip of the plunger against the valve seat, a droplet discharge device for discharging the liquid material in the droplet state by applying inertial force to the liquid material with operations of moving the plunger forward and stopping the forward movement of the plunger, wherein the device includes a plunger position determining mechanism that specifies a position of the tip of the plunger upon the stop of the forward movement to be held near an inner wall of a liquid chamber, which is located ahead in an advancing direction of the plunger (see Patent Document 2).
Furthermore, Patent Document 3 discloses a fluid droplet applying apparatus in which a pressure wave is formed by displacing an end surface of a rod forward and backward inside a chamber through a very small stroke with high acceleration and large force by employing a drive device, and the pressure wave is propagated through a material in the chamber, thus causing the material to be discharged from a nozzle opening.
Meanwhile, with further reduction in size and weight of electronic equipment, reduction in size and weight of electronic components incorporated in the electronic equipment has also been progressed in recent years. For instance, a component having mount dimensions of 400 μm×200 μm, called the “0402 component”, which can greatly reduce a mount area, has been practically mounted since about 2005. The 0402 component is mounted by solder printing using a metal plate at present. However, there is a problem of requiring a contrivance, such as half-etching, in a situation where the 0402 component is present together with large-sized components. Another problem is that a coating amount (coating thickness) has to be controlled individually. For that reason, the mounting with the printing is poor in yield. Moreover, component layout is limited in some cases when good printing performance is to be ensured.
In the droplet discharge device using the plunger that is moved reciprocally, the above-mentioned problems are not caused because the liquid material can be controlled with the operation of the plunger. In that type of device, however, there is not yet realized a technique of high-accurately discharging a droplet of a liquid, such as a solder paste, in a minute volume (e.g., several tens to several hundreds μm in terms of a diameter of the landed droplet), which is required for a small-sized component, without contacting the plunger with the valve seat.